The invention also relates to an irrigation solution, prepared from this product and intended for the irrigation of plants.

State of the art Plants require for their growth heat and sunlight, water and nutrients. The growth of plants is determined by the growth factor that is in the shortest supply relative to their need. To guarantee the growth possibilities, efforts are made to ensure plants a sufficient supply of water and nutrients.

Not only ensuring a sufficient supply of water, but also ensuring a supply of irriga- tion water of specifically the correct quality in terms of plant growth, may be prob- lematic. It is particularly problematic when the soil is calcareous and alkaline. In this case the raw water also contains dissolved calcium and bicarbonate, which ren- der it alkaline and hard, and thus less suitable for the irrigation of plants.

Of the various irrigation methods, drip irrigation wherein the concentrations of nu- trients in the irrigation water can be adjusted to correspond more precisely to the needs of plants at each given time has proven to be efficient with respect to the use of both water and nutrients. For example, the efficiency of phosphorus in drip irri- gation is considerably higher than the efficiency of a solid phosphorus fertilizer ap- plied to the soil by local fertilizing. Drip irrigation has indeed become a popular cultivation method, for example, in the Mediterranean countries.

In drip irrigation, technical problems may be incurred from the use of hard and alka- line water, for example, as calcium carbonate (CaC03) forms precipitations on the walls of the irrigation pipes and the nozzles. This problem is especially emphasized in sub-surface drip irrigation (SDI) pipes.

In practice, attempts have been made to solve this problem by dissolving precipita- tions formed in pipes by means of strong mineral acids, such as nitric acid or phos- phoric acid. Being strong liquid acids they are not only difficult to handle but also hazardous to people and the environment. Solid inorganic acids, such as solid phos-

phoric acid hemihydrate (melting point 29. 3 °C, CRH 9 % RH), are also out of the question because of their disadvantageous physical properties.

From patent application WO 01/25168 there is known a solid water-soluble fertil- izer composition that contains, in addition to fertilizers, also a phosphorus-free or- ganic acid to control the pH of the water. However, in nutrient solutions organic acids may act as nutrients for microbes; there forms slimy microbial growth that clogs the nozzles of the irrigation equipment. Furthermore, organic substances are not compatible with oxidizing substances such as potassium nitrate. Since organic acids are expensive and do not contain nutrients necessary for plants, their use causes only futile expense.

The adding of solid urea phosphate to irrigation fertilizers has been a clear im- provement in preventing the formation of precipitates. Simultaneously with the ad- dition of nitrogen and phosphorus as nutrients to the solution, also the pH of the solution will be adjusted to an acidic value and the formation of precipitates will be presented. Such an option has been disclosed by, for example, R. L. Mikkelsen (Soil Sci. Soc. Am. J. 1987 51: 2 464-468).

From patent publication ZA 7507914 (Triomf Fertilizers Ltd) there is known a method for the preparation of a solid, acid NPK mixed fertilizer, wherein the raw materials are urea, a strong phosphoric acid solution and potassium nitrate.

It is known that the pH of a solution usable for irrigation fertilizing is preferably 5- 7. However, in order for a grower to know how to prepare such a solution for use, he should know the pH and buffering capacity of the raw water, the urea phosphate concentration in the solution, and the effect of the other fertilizers present in the solution on the pH value of the completed solution for use. There is also the addi- tional drawback that the pH of even one and the same fertilizer, for example mono- ammonium phosphate, may vary depending on the preparation process of the prod- uct and the raw materials used therein.

Most irrigation fertilized plants require of the principal nutrients (N, P, K) potas- sium relatively the most, nitrogen slightly less and phosphorus the least. For this reason, potassium nitrate is indeed an ideal nutrient salt for use as a source of nitro- gen and potassium in irrigation fertilization.

Both in the cultivation of garden plants and in greenhouses the most common potas- sium salt even in drip irrigation cultivation is potassium nitrate. Potassium chloride is used less because of its detrimentally high chloride content. Potassium sulfate, for

its part, cannot be used if the raw water contains a large amount of calcium, which precipitates as calcium sulfate, i. e. gypsum.

Potassium nitrate is used either alone or in fertilizer mixtures together with other nutrient salts to provide the desired nutrient proportions. It is also common that growers themselves prepare from different fertilizers the solutions they need for use.

For good growth a plant needs not only water and the principal nutrients N, P and K, but also secondary and trace nutrients. It is known that the availability of both phosphorus and trace nutrients largely depends on the pH (Kluwer Academic Pub- lishers, Fertilizer Manual, 1998, pp. 24-26).

A plant thrives best in an environment having a pH of approx. 5-7. Within this pH range most nutrients are in a form usable by plants. In an alkaline environment there occurs precipitation of secondary and trace nutrients in the form of phosphates or other compounds. At a low pH (pH <5), on the other hand, the concentration of, for example, aluminum in the soil solution increases sharply. This has detrimental ef- fects on plant roots. In general a pH range of 5.5-6. 5 is advantageous for root growth, which in turn is a prerequisite for plant growth (Taiz L. , Zaiger E. , Plant Physiology, 1991, p. 105; Kluwer Academic Publishers, Fertilizer Manual, 1998, p.

24-26).

Drip irrigation fertilization is one of the most efficient forms of fertilization. Prob- lems may, however, appear when precipitates (e. g. CaCO3) are formed from alka- line and hard waters on pipe walls and in nozzles. The problem is emphasized in particular in sub-surface drip irrigation (SDI) pipes.

Description of the invention As a solution to the problems described above it has been invented to add urea phosphate to potassium nitrate. Owing to the sufficient acidity of urea phosphate, the amounts of urea phosphate used are small and do not restrict the use of conven- tional fertilizer compositions in irrigation fertilization. With 0.01-0. 3 % (w/w) aqueous solutions of potassium nitrate acidified with urea phosphate it is possible to adjust the pH of the raw water according to the hardness of the water to a range wherein no CaC03 precipitates are formed and whereby the problems caused by humus and slimy bacterial covers are also eliminated.

According to the present invention, there is thus provided an aqueous solution us- able for irrigation fertilizing of plants, the solution being formed from a solid potas- sium nitrate acidified with urea phosphate and from neutral or alkaline water and containing the said acidic potassium nitrate in an amount of 0.01-0. 3 % by weight of potassium nitrate.

According to the invention there is also provided a solid, acidic potassium nitrate product for controlling the chemical properties of neutral or alkaline water and for preparing an aqueous solution intended for irrigation fertilization of plants, the po- tassium nitrate product comprising 85-98 % by weight of potassium nitrate and 2-15 % by weight of urea phosphate.

Thus the invention relates preferably to a solid, acidic potassium nitrate by means of which the pH and bicarbonate concentration of hard, alkaline water can be adjusted to correspond better to the growth requirements of plants. The acidic potassium ni- trate according to the invention contains at maximum 15 % by weight of urea phos- phate. The invention also relates to an at maximum 0.3 % (w/w) solution for use, containing an acidic potassium nitrate. Acidic potassium nitrate used in accordance with the invention may be a mixture of crystalline materials, a granular product.

The acidic potassium nitrate according to the invention can be prepared, for exam- ple, simply by mixing a crystalline urea phosphate and a crystalline potassium ni- trate at the said weight ratios.

Potassium nitrate acidified with urea phosphate thus has a positive effect on the dis- solving of both phosphorus and secondary and trace nutrients, and it thereby also improves the ability of plants to use these nutrients. However, irrigation solutions containing it are not too acidic, for example, for use in drip irrigation. Also, when the product according to the invention is used, separate equipment associated with the adding of acid are not necessary.

In general, for irrigation fertilization there is first prepared an approx. 10 % (w/w) solution for storage, which is then further diluted to an approx. 0.01-0. 3 % (w/w) solution for use. The acidic potassium nitrate according to the invention, which con- tains urea phosphate 2-15 % (w/w), may also be used as dilutions of 0.01-0. 3 % (w/w), preferably as dilutions of 0.05-0. 3 % (w/w), when the pH of the available raw water is 7-9 and its bicarbonate concentration is 170-350 ppm HC03. The use of the acidic potassium nitrate is especially advantageous with a medium-hard

water having a pH of 7-9 and a bicarbonate concentration of 170-250 ppm HC03.

The use of the product according to the invention for irrigation fertilizing purposes is, however, not limited only to hard water; it is also suitable for use with soft wa- ters as long as the correct dilution proportions are taken into account.

The pH of a solution used in drip irrigation can be adjusted, by means of a 0.01- 0.3 % aqueous solution of potassium nitrate acidified with urea phosphate, to a pH range within which CaC03 precipitates do not form.

Together with the product it is possible to use in irrigation fertilization also other common fertilizers, such as urea and/or monoammonium phosphate (MAP), as long as care is at the same time taken that the total nutrient concentration in the irrigation solution remains below 0.3 % (w/w).

In fertilizer compositions the proportion of potassium nitrate varies, but it is typi- cally approx. one-half or more than one-half of the total amount of the fertilizer.

The adding of urea phosphate to potassium nitrate, i. e. the acidification of potas- sium nitrate, thus makes it possible to regulate the pH of conventional fertilizer so- lutions (nutrient concentration <0.3 %; KN03 over 50 % (w/w) of the solids) used in irrigation fertilization. The acidic potassium nitrate solution automatically adjusts to the correct level the pH of the solution used where medium hard water is avail- able. Such areas exist in, for example, the Mediterranean countries, South America and Australia.

Owing to its low phosphorus content, the use of the acidic potassium nitrate is pos- sible also when the phosphorus requirement of a plant is low. If, on the other hand, the phosphorus requirement of a plant is higher, any phosphorus fertilizer suitable for irrigation fertilization can be used for additional fertilization.

Table 1 shows the nutrient composition of the acidic potassium nitrate according to the invention as the amount of urea phosphate (UP) present in it varies.

Table 1 Nutrient proportions in acidic potassium nitrate In the mixture % In the mixture Relative wt. UP and % wt. N-P205-K20 N-P205-K20 KN03 UP KN03 N P205 K20 N P205 K20 15 85 14.3 6.6 39.1 1.0 0.5 2.7 10 90 14.1 4.4 41.4 1.0 0.3 2.9 5 95 13.9 2.2 43.7 1.0 0.2 3.1 0 100 13.7 0.0 46.0 1.0 0.0 3.4

The other advantages of the acidic potassium nitrate include a good physical qual- ity, since both components absorb moisture only a little or not at all.

The corrosive action of urea phosphate on metals (e. g. copper and zinc metals) is also considerably less than that of the mineral acids used for corresponding pur- poses. The advantageousness of urea phosphate is emphasized when the solution contains a large amount of potassium nitrate (Corros. Sci. (1976), 16 (8), 485-98).

Compared with other mineral acids, phosphoric acid has a strongly buffering char- acter. In practice this means that, when a potassium nitrate acidified with urea phos- phate is added, the pH of alkaline water (e. g. pH 8) can be lowered rapidly and ef- fectively to the desired range (< pH 7), but owing to the buffer effect the pH will, however, remain within a range of pH 5-7.

For the pH control of especially hard waters (e. g. 250-350 ppm HCO3) it is possi- ble to use either stronger solutions of acidic potassium nitrate (e. g. 0.2-0. 3 % (w/w) ) or an acidic potassium nitrate containing relatively more urea phosphate.

The invention is defined in greater detail in claims 1-10.

The following examples are intended for elucidating the invention without limiting it in any manner.

Example 1 Comparison of acidic potassium nitrate solutions Two fertilizer solution series were prepared in ion-exchanged water, one series con- taining potassium nitrate acidified with urea phosphate and the other potassium ni- trate acidified with monoammonium phosphate.

The pH values of the solutions were measured. The results of the measurements are shown in Table 2.

Table 2 Concentration of Potassium nitrate Potassium nitrate Potassium nitrate fertilizer solution, (90 %) and urea (90 %) and mono- (100 %), pH % by weight phosphate (10 %), ammonium phos- pH phate (10 %), pH 0. 1 3. 22 5. 59 6. 87 1. 0 2. 35 4. 83 6. 85 10. 0 1. 55 4. 30 6. 82

The results show that, when the same amounts of monoammonium phosphate and urea phosphate are added to potassium nitrate, a considerably greater pH change is obtained with potassium nitrate acidified with urea phosphate than with potassium nitrate acidified with monoammonium phosphate.

Example 2 Effect of acidic potassium nitrate on medium-hard water A synthetic, medium-hard water having a bicarbonate concentration of 175 ppm HC03, a calcium concentration of 82 ppm Ca and a pH of 7.2 was prepared by add- ing 241 mg of sodium bicarbonate and 160 mg of calcium chloride to one liter of ion-exchanged water.

From five potassium nitrates acidified with different urea phosphate amounts a se- ries of solutions was prepared in the synthetic, medium-hard water mentioned above. The reference solutions were prepared from pure potassium nitrate. The pH values of the solutions were measured. The results of the measurements are shown in Table 3.

Table 3 Composition of fertil-0. 05 % 0. 1 % 0. 2 % 0. 3 % izer nutrient so-nutrient so-nutrient so-nutrient so- lution lution lution lution H pH pH pH 15 % UP, 85 % KNO3 7. 0 7. 0 6. 4 5. 6 12 % UP, 88 % KNO3 7. 0* 6. 5 6. 0 8 % UP, 92% KNO3 6. 9 6. 5 5 % UP, 95 % KNO3 - - - 6.9 2% UP, 98 % KNO3 - - - 7. 0* 0% UP, 100 % KN03 7.8 7.9 8.1 8.1 (*pH of the raw water approx. 7)

Example 3 Effect of acidic potassium nitrate on medium-hard water A synthetic, medium-hard water having a bicarbonate concentration of 250 ppm HC03, a calcium concentration of 82 ppm Ca, and a pH of 7 was prepared by add- ing 344 mg of sodium bicarbonate and 228 mg of calcium chloride to one liter of ion-exchanged water.

From four potassium nitrates acidified with different urea phosphate amounts a se- ries of solutions were prepared in this water. The solution concentrations were within the range 0.1-0. 3 %. Reference solutions containing only potassium nitrate were prepared. The pH values of the solutions were measured, and the results are shown in Table 4.

Table 4

Composition of fertil-0. 1 % 0. 2 % 0. 3 % izer nutrient solution nutrient solution nutrient solution pH pH pH 15 % UP, 85 % KNO3 7. 0 6. 6 6. 1 12 % UP, 88 % KNO3 - 6.7 6.4 8 % UP, 92 % KN03 7. 0 6. 7 5 % UP, 95 % KNO3 - - 7.0 0 % UP, 100 % KN03 8. 0 8. 0 8. 0

Example 4 Effect of acidic potassium nitrate on rather hard water A synthetic, rather hard water having a bicarbonate concentration of 300 ppm HC03, a calcium concentration of 99 ppm Ca and a pH of 7 was prepared by adding 413 mg/liter of sodium bicarbonate and 274 mg/liter of calcium chloride to ion- exchanged water.

From four potassium nitrates acidified with different urea phosphate amounts solu- tions of 0.2-0. 3 % were prepared in this water. Reference solutions containing only potassium nitrate were prepared. The pH values of the solutions were meas- ured, and the results are shown in Table 5.

Table 5 Composition of fertilizer 0. 2% 0. 3 % nutrient solution nutrient solution pH pH 15 % UP, 85 % KNO3 6. 8 6. 5 12 % UP, 88 % KNO3 6. 9 6. 7 8 % UP, 92 % KN03 6. 9 5% UP, 95% KNO3 7. 1 0% UP, 100 % KN03 7. 8 7. 8 Example 5 Effect of acidic potassium nitrate on rather hard water A synthetic, rather hard water having a bicarbonate concentration of 330 ppm HC03, a calcium concentration of 109 ppm Ca and a pH of 7 was prepared by add- ing 454 mg/liter of sodium bicarbonate and 301 mg/liter of calcium chloride to ion- exchanged water.

From two potassium nitrates acidified with different urea phosphate amounts solu- tions of 0.05-0. 3 % were prepared in this water. Reference solutions containing only potassium nitrate were prepared. The pH values of the solutions were meas- ured, and the results are shown in Table 6.

Table 6 Composition of fertil-0. 05 % 0. 2 % 0. 3 % izer nutrient solution nutrient solution nutrient solution pH pH pH 15 % UP, 85 % KNO3 7. 1 6.9 6.7 12% UP, 88 % KNO3 - 7.0 6.5 0% UP, 100 % KN03 7. 6 7.9 8.0

On the basis of the tests performed it can be stated that with the help of acidified potassium nitrate it was possible to adjust the acidity of waters of different levels of hardness (pH approx. 7) to a pH suitable for the irrigation of plants. Reference solu- tions containing only potassium nitrate surprisingly raised the pH of water to 8.